Temperature monitoring return electrode

ABSTRACT

An electrosurgical return electrode includes a conductive pad including a patient-contacting surface configured to conduct electrosurgical energy, and a temperature sensing circuit coupled to the conductive pad. The temperature sensing circuit includes a plurality of switching elements connected in series, wherein each of the plurality of switching elements is configured to vary in impedance in response to temperature changes, such that an interrogation signal transmitted therethrough is indicative of the temperature change.

BACKGROUND

1. Technical Field

The present disclosure relates to electrosurgical apparatuses, systemsand methods. More particularly, the present disclosure is directed toelectrosurgical systems utilizing one or more return electrodesconfigured to monitor temperature.

2. Background of Related Art

Energy-based tissue treatment is well known in the art. Various types ofenergy (e.g., electrical, ultrasonic, microwave, cryo, heat, laser,etc.) may be applied to tissue to achieve a desired surgical result.Electrosurgery typically involves application of high radio frequencyelectrical current to a surgical site to cut, ablate, coagulate or sealtissue. In monopolar electrosurgery, a source or active electrodedelivers radio frequency energy from the electrosurgical generator tothe tissue and a return electrode carries the current back to thegenerator. In monopolar electrosurgery, the source electrode istypically part of the surgical instrument held by the user and appliedto the tissue to be treated. The patient return electrodes are typicallyin the form of pads adhesively adhered to the patient and are placedremotely from the active electrode to carry the current back to thegenerator.

The return electrodes usually have a large patient contact surface areato minimize heating at that site. Heating is caused by high currentdensities which directly depend on the surface area. A larger surfacecontact area results in lower localized heat intensity. Returnelectrodes are typically sized based on assumptions of the maximumcurrent utilized during a particular surgical procedure and the dutycycle (i.e., the percentage of time the generator is on).

The first types of return electrodes were in the form of large metalplates covered with conductive jelly. Later, adhesive electrodes weredeveloped with a single metal foil covered with conductive jelly orconductive adhesive. However, one problem with these adhesive electrodeswas that if a portion peeled from the patient, the contact area of theelectrode with the patient decreased, thereby increasing the currentdensity at the adhered portion and, in turn, increasing the heating atthe tissue. This risked burning the patient in the area under theadhered portion of the return electrode if the tissue was heated beyondthe point where circulation of blood could cool the skin.

To address this problem various return electrodes and hardware circuits,generically called Return Electrode Contact Quality Monitors (RECQMs),were developed. Such systems relied on measuring impedance at the returnelectrode to calculate a variety of tissue and/or electrode properties.These systems were only configured to measure changes in impedance ofthe return electrodes to detect peeling.

SUMMARY

The present disclosure relates to electrosurgical return electrodes.Disclosure provides for an electrosurgical return electrode pad havingvarious mechanisms and/or circuits adapted to prevent tissue damage tothe patient. The return electrode pad monitors temperature and upon thetemperature reaching a predetermined threshold the pad activates safetycircuits and/or alarms. Further, a modular smart return electrode isdisclosed having a disposable portion and a detachable portion includingsensor and/or control circuits.

According to one aspect of the present disclosure, an electrosurgicalreturn electrode is provided. The return electrode includes a first andsecond flexible conductive material layers, wherein impedance of thefirst and second conductive material layers in continuously monitoredand a material layer disposed between the first and second conductivematerial layers. The material layer is transmitionable between a solidstate and a non-solid state, the material layer is also configured tomelt upon an increase in temperature beyond a predetermined threshold,thereby increasing conductivity between the first and second conductivematerial layer.

According to a further aspect of the present disclosure, anelectrosurgical return electrode is disclosed. The return electrodeincludes a return electrode pad having a patient-contacting surfaceconfigured to conduct electrosurgical energy. The return electrode alsoincludes a material layer disposed on the return electrode pad. Thematerial layer includes at least one switching element having at leastone switch component formed from a shape memory material thattransmissions from an austensitic state to a martensitic state upon achange in temperature to actuate at least one switch component of the atleast one switching element.

According to another embodiment of the present disclosure, anelectrosurgical return electrode is disclosed. The return electrodeincludes a return electrode pad having a patient-contacting surfaceconfigured to conduct electrosurgical energy and a ferromagneticmaterial layer disposed on the return electrode pad. The ferromagneticmaterial layer includes a magnetic field and at least one temperaturesensing switch, the ferromagnetic material layer is configured to lose amagnetic flux at a predetermined temperature threshold thereby actuatingthe at least one temperature sensing switch.

According to a further embodiment of the present disclosure anelectrosurgical return electrode is disclosed. The return electrodeincludes a conductive pad including a patient-contacting surfaceconfigured to conduct electrosurgical energy and a temperature sensingcircuit coupled to the conductive pad. The temperature sensing circuitincludes a plurality of switching elements connected in series, whereineach of the plurality of switching elements is configured to vary inimpedance in response to temperature changes. An interrogation signal istransmitted therethrough measures impedance and is indicative of thetemperature change.

An electrosurgical return electrode is also contemplated by the presentdisclosure. The return electrode includes a conductive pad including apatient-contacting surface configured to conduct electrosurgical energyand a temperature sensing circuit coupled to the conductive pad. Thetemperature sensing circuit includes a thermocouple matrix having afirst plurality of metallic wires and a second plurality of metallicwires intersecting at a plurality of junctions, wherein each of thejunctions is interrogated to obtain a temperature measurement.

According to another embodiment of the present disclosure, a modularelectrosurgical return electrode is disclosed. The return electrodeincludes a disposable portion having a patient-contacting surfaceconfigured to conduct electrosurgical energy and a detachable substratehaving a sensor circuit configured to monitor at least one of a returnelectrode pad property and a tissue property to generate sensor data.The detachable substrate is configured to couple to the disposableportion.

According to a further embodiment of the present disclosure, anelectrosurgical return electrode is disclosed. The return electrodeincludes a conductive pad including a patient-contacting surfaceconfigured to conduct electrosurgical energy and a temperature sensingcircuit coupled to the conductive pad. The temperature sensing circuitincludes at least one transistor coupled to a multiplexer, the at leastone transistor having a predetermined forward voltage drop that isindicative of temperature of the conductive pad.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present disclosure are described herein withreference to the drawings wherein:

FIG. 1 is a schematic block diagram of an electrosurgical systemaccording to the present disclosure;

FIG. 2 is a schematic block diagram of a generator according to oneembodiment of the present disclosure;

FIG. 3 is a top view of the electrosurgical return electrode of themonopolar electrosurgical system of FIG. 1;

FIG. 4 is a cross-sectional side view of an electrosurgical returnelectrode having a positive temperature coefficient (PTC) material andadhesive material layers;

FIG. 5 is a cross-sectional side view of an electrosurgical returnelectrode having a material layer according to the present disclosure;

FIGS. 6A-C show an electrosurgical return electrode having a shapememory material layer according to the present disclosure;

FIG. 7 is a cross-sectional side view of an electrosurgical returnelectrode having a ferromagnetic material layer according to the presentdisclosure;

FIGS. 8A-C illustrate one embodiment of an electrosurgical returnelectrode having a temperature sensing circuit according to the presentdisclosure;

FIGS. 9A-B illustrate one embodiment of an electrosurgical returnelectrode having a temperature sensing circuit according to the presentdisclosure;

FIG. 10A-B illustrate one embodiment of an electrosurgical returnelectrode having a temperature sensing circuit according to the presentdisclosure;

FIG. 11 is a cross-sectional plan view of a smart electrosurgical returnelectrode having temperature sensing circuit according to the presentdisclosure; and

FIG. 12 is a cross-sectional side view of a modular electrosurgicalreturn electrode according to the present disclosure.

DETAILED DESCRIPTION

Particular embodiments of the present disclosure are describedhereinbelow with reference to the accompanying drawings. In thefollowing description, well-known functions or constructions are notdescribed in detail to avoid obscuring the present disclosure inunnecessary detail.

FIG. 1 is a schematic illustration of an electrosurgical systemaccording to one embodiment of the present disclosure. The systemincludes an electrosurgical instrument 2 having one or more electrodesfor treating tissue of a patient P. The instrument 2 is a monopolarinstrument including one or more active electrodes (e.g.,electrosurgical cutting probe, ablation electrode(s), etc.).Electrosurgical RF energy is supplied to the instrument 2 by a generator20 via an electrosurgical cable 4, which is connected to an activeoutput terminal, allowing the instrument 2 to coagulate, ablate and/orotherwise treat tissue. The energy is returned to the generator 20through a return electrode 6 via a return cable 8. The system mayinclude a plurality of return electrodes 6 that are arranged to minimizethe chances of tissue damage by maximizing the overall contact area withthe patient P. In addition, the generator 20 and the return electrode 6may be configured for monitoring so-called “tissue-to-patient” contactto insure that sufficient contact exists therebetween to furtherminimize chances of tissue damage.

The generator 20 includes input controls (e.g., buttons, activators,switches, touch screen, etc.) for controlling the generator 20. Inaddition, the generator 20 may include one or more display screens forproviding the user with variety of output information (e.g., intensitysettings, treatment complete indicators, etc.). The controls allow theuser to adjust power of the RF energy, waveform, and other parameters toachieve the desired waveform suitable for a particular task (e.g.,coagulating, tissue sealing, intensity setting, etc.). The instrument 2may also include a plurality of input controls that may be redundantwith certain input controls of the generator 20. Placing the inputcontrols at the instrument 2 allows for easier and faster modificationof RF energy parameters during the surgical procedure without requiringinteraction with the generator 20.

FIG. 2 shows a schematic block diagram of the generator 20 having acontroller 24, a high voltage DC power supply 27 (“HVPS”) and an RFoutput stage 28. The HVPS 27 provides high voltage DC power to an RFoutput stage 28, which then converts high voltage DC power into RFenergy and delivers the RF energy to the active electrode. Inparticular, the RF output stage 28 generates sinusoidal waveforms ofhigh RF energy. The RF output stage 28 is configured to generate aplurality of waveforms having various duty cycles, peak voltages, crestfactors, and other suitable parameters. Certain types of waveforms aresuitable for specific electrosurgical modes. For instance, the RF outputstage 28 generates a 100% duty cycle sinusoidal waveform in cut mode,which is best suited for ablating, fusing and dissecting tissue, and a1-25% duty cycle waveform in coagulation mode, which is best used forcauterizing tissue to stop bleeding.

The controller 24 includes a microprocessor 25 operably connected to amemory 26, which may be volatile type memory (e.g., RAM) and/ornon-volatile type memory (e.g., flash media, disk media, etc.). Themicroprocessor 25 includes an output port that is operably connected tothe HVPS 27 and/or RF output stage 28 that allows the microprocessor 25to control the output of the generator 20 according to either openand/or closed control loop schemes. Those skilled in the art willappreciate that the microprocessor 25 may be substituted by any logicprocessor (e.g., control circuit) adapted to perform the calculationsdiscussed herein.

A closed loop control scheme is a feedback control loop wherein sensorcircuit 22, which may include a plurality of sensors measuring a varietyof tissue and energy properties (e.g., tissue impedance, tissuetemperature, output current and/or voltage, etc.), provides feedback tothe controller 24. Such sensors are within the purview of those skilledin the art. The controller 24 then signals the HVPS 27 and/or RF outputstage 28, which then adjust DC and/or RF power supply, respectively. Thecontroller 24 also receives input signals from the input controls of thegenerator 20 or the instrument 2. The controller 24 utilizes the inputsignals to adjust power outputted by the generator 20 and/or performsother control functions thereon.

FIGS. 3 and 4 illustrate various embodiments of the return electrode 6for use in monopolar electrosurgery. The return electrode 6 includes areturn electrode pad 30 having a top surface and a patient-contactingsurface 32 configured to receive current during monopolarelectrosurgery. The patient-contacting surface 32 is made from asuitable conductive material such as metallic foil. While FIG. 3 depictsthe return electrode 6 in a general rectangular shape, it is within thescope of the disclosure for the return electrode 6 to have any suitableregular or irregular shape.

Referring to FIG. 4, another embodiment of the return electrode 6 isshown, wherein the return electrode pad 30 includes a positivetemperature coefficient (PTC) material layer 38 deposited thereon. ThePTC material 38 can be made of, inter alia, a polymer/carbon-basedmaterial, a cermet-based material, a polymer material, a ceramicmaterial, a dielectric material, or any combinations thereof. The PTCmaterial layer 38 acts to distribute the temperature created by thecurrent over the surface of the electrosurgical return electrode 6,which minimizes the risk of a patient burn. The PTC material of thelayer 38 may be replaced by a negative temperature coefficient material.These materials change resistance dramatically when heated, allowing formonitoring of impedance to detect a rise in temperature, or automaticchanges in localized current densities, thereby reducing localizedheating.

The return electrode 6 further includes an adhesive material layer 39 onthe patient-contacting surface 32. The adhesive material can be, but isnot limited to, a polyhesive adhesive, a Z-axis adhesive, awater-insoluble, hydrophilic, pressure-sensitive adhesive, or anycombinations thereof, such as POLYHESIVE™ adhesive manufactured byValleylab of Boulder, Colo. The adhesive may be conductive and/ordielectric. The adhesive material layer 39 ensures an optimal surfacecontact area between the electrosurgical return electrode 6 and thepatient “P,” which limits the possibility of a patient burn. In anembodiment where PTC material layer 38 is not utilized, the adhesivematerial layer 39 may be deposited directly onto the patient-contactingsurface 32.

Referring to FIG. 5, another embodiment of the return electrode 6 isshown, wherein the return electrode pad 30 includes first and secondfoil layers 72 and 74, respectively, and a material layer 70 disposedtherebetween. The return electrode 6 also includes the adhesive materiallayer 39 on the patient-contacting surface 32. The foil layers 72 and 74are made from a suitable conductive material (e.g., metal) adapted toconduct the electrosurgical energy from the surgical site to thegenerator 20. The material layer 70 acts as a dielectric material andseparates the foil layers 72 and 74 from each other by a predetermineddistance (e.g., thickness of the material layer 70) thereby isolatingthe foil layers 72 and 74 and reducing the overall conductivity of thereturn electrode pad 30.

The material layer 70 is formed from a material that changes from asolid state to a non-solid state (e.g., liquid) at a predeterminedtemperature such as wax, temperature sensitive gel, jelly, plastic,various fats, low molecular weight organic compounds, polymer hydrogeland the like. The material of the material layer 70 has a melting pointthat approximately coincides with the maximum safe temperature—thetemperature above which tissue damage starts to occur. A maximum safetemperature may be from about 40° C. to about 45° C. During normaloperating temperatures (e.g., room temperature of about 20° C., bodysurface temperature of approximately 37° C.) the material layer 70 is insolid state but is flexible enough to allow for maximum conformity ofthe return electrode pad 30 to the patient.

When the return electrode pad 30 reaches the maximum safe temperature,the material layer 70 begins melting and undergoes a transmission fromthe solid state to a liquid state. As a result, the foil layers 72 and74 come into electrical contact thereby increasing conductiontherebetween. During operation, the impedance of the foil layers 72 and74 is monitored. With increased conduction between the foil layers 72and 74 impedance of the return electrode 30 drops. Monitoring the dropin impedance of the return electrode pad 30 allows the generator 20 toprovide a temperature increase warning and/or adjust RF energy supply(e.g., shut off).

In another embodiment, when the material layer 70 is in a solid state,it provides an electrical interface between foil layers 72 and 74. Whenthe material layer 70 melts, it would then increase resistance betweenfoil layers 72 and 74 or break the electrical contact between foillayers 72 and 74. This change, in turn, could be utilized to provide atemperature increase warning and/or adjust RF energy supply (e.g., shutoff). In still other embodiments, depending on the type of material usedfor material layer 70, the dielectric properties of material layer 70could change with temperature, thus changing the value of capacitancebetween foil layers 72 and 74.

Referring to FIGS. 6A-C, another embodiment of the return electrode 6 isshown, wherein a shape memory material layer 76 is disposed on top ofthe return electrode pad 30 (e.g., the foil layer 72). The shape memorymaterial layer 76 may be formed from a suitable shape memory alloyincluding shape memory metals such as nitinol, flexinol and the like.Shape memory materials change conformation (e.g., shape) atpredetermined temperatures. The temperature at which the shape memorymaterial reverts to its conformation is controlled by varying the ratioof shape memory materials of the alloy. The shape memory material of thelayer 76 is configured to change shape at or around a maximum safetemperature, which may be from about 40° C. to about 45° C.

More particularly, shape memory alloys (SMAs) are a family of alloyshaving anthropomorphic qualities of memory and trainability. SMAs havebeen applied to such items as actuators for control systems, steerablecatheters and clamps. One of the most common SMAs is Nitinol, which canretain shape memories for two different physical configurations andchanges shape as a function of temperature Recently, other SMAs havebeen developed based on copper, zinc and aluminum and have similar shapememory retaining features.

SMAs undergo a crystalline phase transmission upon applied temperatureand/or stress variations. A particularly useful attribute of SMAs isthat after it is deformed by temperature/stress, it can completelyrecover its original shape on being returned to the originaltemperature. The ability of an alloy to possess shape memory is a resultof the fact that the alloy undergoes a reversible transformation from anaustensitic state to a martensitic state with a change intemperature/stress. This transformation is referred to as athermoelastic martensitic transformation.

Under normal conditions, the thermoelastic martensitic transformationoccurs over a temperature range, which varies with the composition ofthe alloy, itself, and the type of thermal-mechanical processing bywhich it was manufactured. In other words, the temperature at which ashape is “memorized” by an SMA is a function of the temperature at whichthe martensite and austenite crystals form in that particular alloy. Forexample, Nitinol alloys can be fabricated so that the shape memoryeffect may occur over a wide range of temperatures, e.g., −270° to +100°Celsius.

Many SMAs are also known to display stress-induced martensite (SIM)which occurs when the alloy is deformed from its original austensiticstate to a martensitic state by subjecting the alloy to a stresscondition.

The shape memory material layer 76 is constructed as an array ofswitching elements 78 as shown in FIGS. 6B-C. The switching elements 78may be disposed on a suitable flex circuit (not explicitly shown) suchas a flexible holding substrate 48 shown in FIG. 8B manufactured from asuitable substrate. The current flow to the switching elements 78 isprovided by a power source (not explicitly shown), such as a low voltageDC power source (e.g., battery, AC/DC transformer, power source 50 ofFIG. 8A etc.). The switching elements 78 include first and second switchcomponents 77 and 79 that are formed from the shape memory material ofthe layer 76. The switch components 77 and 79 form a circuit around aconnector 80, which may be a portion of the foil layer 76, flex circuitor any conducting strip of metal.

Upon a change in temperature, the SMA of the material layer 76transmissions from an austensitic state to a martensitic state toactuate one or more of the switch components 77 and 79 of the switchingelement 78. More specifically, at normal operating temperatures (e.g.,up to 40° C.), the first and second switch components 77 and 79 are infirst configuration in relation to the connector 78. In the firstconfiguration, the switch components 77 and 79 are arranged in a closedcircuit, when the first and second components 77 and 79 are in contactwith the connector 78, in which case in the second configuration theswitch components 77 and 79 conform to create an open circuit. When thetemperature changes due to over heating of the return electrode pad 30,the switch components 77 and 79, change to the second configuration,since the components are formed from a shape memory material. The firstconfiguration may also be an open circuit configuration, and hence, inthe second configuration, the switch components 77 and 79 are adapted todeform into a closed circuit configuration.

During operation, a low DC voltage signal (e.g., from DC power source)is passed through the switching elements 78 to determine the statethereof, e.g., whether the circuit is open or closed. When switchingfrom the first to the second configuration occurs, the change in statusof the switching elements 78 is indicative of a temperature increaseabove the maximum safe temperature. The status change is transmitted tothe generator 20, wherein an alarm is generated or an adjustment to theRF supply is made.

FIG. 7 shows an embodiment of the return electrode 6 including aferromagnetic material layer 82 being disposed on top of the returnelectrode pad 30 (e.g., the foil layer 72). The ferromagnetic material(e.g., liquid) exhibits spontaneous magnetization below a specifictemperature point, the so-called Curie temperature. Above thistemperature, the ferromagnetic material ceases to spontaneously exhibitmagnetization. A ferromagnetic material that has a Curie temperature ator about a maximum safe temperature (e.g., 45° C.) such as manganesearsenic may be used, such that when temperature increases beyond themaximum safe temperature, the ferromagnetic material loses the magneticfield. The termination of the magnetic field is used as an activationmechanism for actuating one or more temperature sensing switches 84. Thetemperature sensing switches 84 may be magnetic field activatableswitches, such as reed switches.

The temperature sensing switches 84 may be disposed on a flex circuit(not explicitly shown), such as a flexible holding substrate 48 shown inFIG. 8B manufactured from a suitable substrate. The current flow to thetemperature sensing switches 84 is provided by a power source (notexplicitly shown), such as a low voltage DC power source (e.g., battery,AC/DC transformer, power source 50 of FIG. 8A etc.).

The temperature sensing switches 84 are configured to remain open whenthe magnetic field is active (e.g., temperature of the return electrodepad 6 is below the maximum safe temperature) and close once the fieldterminates. The temperature sensing switches 84 may be substituted bymagnetic field sensors (not explicitly shown) that are configured tomeasure changes in the magnetic field generated by the ferromagneticmaterial layer 82. This allows for more precise determination oftemperature of the return electrode pad 30 as opposed to a binarysensing system discussed above with respect to the switches 84. Thechange in status of the temperature sensing switches 84 and/or magneticfield measurements corresponding to temperature changes are transmittedto the generator 20, wherein appropriate actions are performed.

FIGS. 8A-C shows the return electrode 6 including a temperature sensingcircuit 40 disposed therein. The temperature sensing circuit 40 includesone or more temperature sensor arrays 41 and 43 having at least onetemperature sensor. Contemplated temperature sensors includethermocouples, thermistors, semiconductor (e.g., silicon) diodes,transistors, ferrite materials and Hall effect devices. The temperaturesensing circuit 40 is disposed on a flex circuit (e.g., a flexibleholding substrate 48) manufactured from suitable substrate, such as apolyimide film. Examples are films sold under the trademarks MYLAR™ andKAPTON™ and the like.

The diodes 42 are connected in series with one or more current limitingresistors 44 and are utilized as temperature sensors. The resistor 44 iscoupled in series with the diode 42, having a resistance selected to setand limit the current flowing through the diode 42 at a predeterminedlevel. The current flow to the diodes 42 is provided by a power source50, such as a low voltage DC power source (e.g., battery, AC/DCtransformer, etc.) connected in series with the diodes 42 and resistors44 via interconnection wires 46. The power source 50 may be integratedinto the generator 20 and draw power from the same source as the HVPS 27(e.g., AC outlet). In one embodiment, interconnection of the diodes 42and the resistors 44 is achieved by deposition of metal traces on theholding substrate 48 and soldering of the diodes 42 and the resistors 44directly into the holding substrate 48. The holding substrate 48 mayalso electrically insulate the temperature sensing circuit 40 from thepatient-contacting surface 32 to prevent RF energy being returned to thegenerator 20 from interfering with the circuit components.

The diodes 42 are so called “forward biased” such that current flowsinitially through the resistor 44 and from the diode's anode to thediode's cathode. In a forward biased diode 42, forward voltage drop (Vf)is produced that is in the range of about 0.5V to about 5V depending onthe type of diode (e.g., light emitting diode). The forward voltage isdirectly dependent on the temperature. In particular, as the temperatureincreases, the semiconductor material within the diode 42 undergoeschanges in their valence and conduction bands and consequently Vfdecreases. Thus, by keeping the current flowing through the diode 42constant via the resistor 44 and measuring the forward bias voltageallows for determination of the temperature of the diode 42.

The Vf signal is transmitted through the interconnection wires 46 to thegenerator 20, wherein the sensor circuit 22 analyzes the Vf to determinea corresponding temperature value. As those skilled in the art willappreciate, each of the interconnection wires 46 may include acorresponding isolation circuit (e.g., optical couplers) to translateelectric signals (e.g., Vf) across isolation barriers, thereby isolatingthe temperature sensing circuit 40 from the RF supply.

The analysis process may include passing the Vf signals through ananalog-to-digital converter and then multiplying the digitized Vf signalby a predetermined factor to arrive at a corresponding temperaturevalue. The factor is derived empirically taking into considerationelectrical properties of the diode 42, resistor 44 as well as electricalproperties of the current being passed therethrough. The temperaturesignal is then transmitted to the controller 24 where it is furtheranalyzed to determine appropriate action. For instance, comparingtemperature measurements with a predetermined temperature threshold andadjusting or terminating the RF energy supply if the temperaturemeasurement is larger than the predetermined threshold.

Temperature across the patient-contacting surface 32 may vary due to anumber of factors (e.g., moisture content, adherence, etc.) affectingcurrent density. Therefore, it may be desirable to measure temperaturesat various points in the return electrode pad 30. Measuring temperatureat various points allows for pinpointing the location of so-called “hotspots,” segments of the patient-contacting surface 32 where currentdensity exceeds that of the surrounding area and results in pad burn.Since measurement of Vf for each diode 42 provides for determination ofcorresponding temperature at the location of the diode 42, placing thediodes 42 strategically within the return electrode pad 30 allows formonitoring of temperature at those locations.

With reference to FIG. 8A, each resistor 44 and diode 42 pair isdisposed within the conducting pad 30 such that the diode 42 providestemperature readings for a corresponding temperature monitoring zone 45.The size of the monitoring zone 45 depends on the distance between thediodes 42. The return electrode pad 30 may include any number ofmonitoring zones 45 of varying sizes. Each diode 42 is identified by thesensor circuit 22 as being associated with a particular monitoring zone45 such that, when Vf signals are transmitted and subsequently convertedinto temperature readings, the generator 20 provides temperaturemonitoring for each of the monitoring zones 45. This data is utilized toinstruct the user which specific portion of the return electrode pad 30includes a hot spot so that preventative action may be taken, ifnecessary. This may include automatic RF supply termination and/oradjustment or manual termination of RF supply to ensure that the returnelectrode pad 30 adheres properly to the patient at the identified hotspot.

FIG. 8C shows another embodiment of return electrode 6 including atemperature sensing circuit 40 disposed on the return electrode pad 30.The temperature sensing circuit 40 includes one or more transistors 150,each of which is connected to a multiplexer 152. Similar to the diodes42, the transistors 150 measure temperature as a function of the voltagedrop which occurs as a result of temperature variation at the returnelectrode pad 30. The multiplexer 152 controls voltage flow totransistors 150, which in response to the voltage signal transmit areturn voltage signal representative of the temperature of the returnelectrode pad 30. The transistors 150 are coupled to the multiplexer 152via a bus 154. The voltage from the transistors 150 is then transmittedto the sensor circuitry 22 of the generator 20 which may be atemperature sensor.

FIGS. 9A-B shows an embodiment of return electrode 6 that includes atemperature sensing circuit 90 disposed on top of the return electrodepad 30. The temperature sensing circuit 90 includes a plurality ofswitching elements 92 adapted to change impedance in response totemperature changes. One suitable type of switching elements 92 is apolymeric positive temperature coefficient overcurrent protection(PPTCOP) device, such as POLYSWITCH™, available from Raychem, a divisionof Tyco Electronics Corporation, located in Menlo Park, Calif. Theswitching elements 92 are wired in series and are disposed on a topsurface of the return electrode pad 30 (e.g., on top of the foil layer72).

The switching elements 92 may also be disposed on a suitable flexcircuit (not explicitly shown), such as the flexible holding substrate48 shown in FIG. 8B manufactured from a suitable substrate. The currentflow to the switching elements 92 is provided by a power source (notexplicitly shown), such as a low voltage DC power source (e.g., battery,AC/DC transformer, power source 50 of FIG. 8A etc.).

During operation, an interrogatory signal (e.g., from power source 50)is transmitted through the temperature sensing circuit 90 (e.g.,interrogation current and/or voltage) to measure impedance and/orcurrent of the switching elements 92. Measuring impedance of thetemperature circuit 90 allows for determination of correspondingtemperature of the return electrode pad 30. PPTCOP devices areconfigured to switch off, thereby increasing impedance, at differentrates depending on the temperature. More specifically, at lowertemperatures (e.g., about 40° C.) the PPTCOP devices turn off at aslower rate than at higher temperatures (e.g., about 45° C.). Therefore,duration of interrogation signals is adjusted to be of sufficient lengthto make sure that the sensor circuit 90 is interrogated properly. Inother words, an interrogation signal at lower temperatures is longersince the activation time of the PPTCOP devices is slower than at highertemperatures, at which the interrogation signal is shorter.

The interrogation signal provides an interrogation circuit (e.g.,generator 20) with the impedance measurement of the temperature sensingcircuit 90. The generator 20 converts the impedance data tocorresponding temperature data and if the temperature is above apredetermined threshold (e.g., 45° C.), the generator 20 performs one ormore appropriate actions (e.g., issues an alarm).

With reference to FIGS. 10A-B, another embodiment of the returnelectrode 6 having a temperature sensing circuit 100 disposed on top ofthe return electrode pad 30 is shown. The temperature sensing circuit100 includes a thermocouple matrix 102 that allows for monitoring oftemperature across an entire surface of the return electrode pad 30. Thethermocouple matrix 102 includes a plurality of metallic filaments orwires disposed on top of the foil layer 72. More specifically, thethermocouple matrix 102 includes a first plurality wires 104 laid in afirst direction across the top surface of the foil layer 72 and a secondplurality of wires 106 laid in a second direction, which issubstantially perpendicular to the first direction. The first pluralityof wires 104 are made from copper and the like and the second pluralityof wires 106 are made from constantan and the like. Each of the firstplurality of wires 104 intersects with each of the second of pluralityof wires 106 at a plurality of junctions 108 thereby forming thethermocouple matrix 102. The junction 108 of first and second pluralityof wires 104 and 106 create a so-called “Seebeck effect.” The voltageproduced at the junction 108 of two dissimilar metals changes directlywith temperature. Different wire type are chosen based on the metal'sability to form different thermocouple junctions which are known toexhibit a given voltage at a given temperature. Following combination ofwires as shown in Table 1 may be used.

TABLE 1 TYPE E TYPE J TYPE K TYPE R TYPE S TYPE T First 10% Nickel Iron10% Nickel 13% Platinum 10% Platinum Copper wires Chromium ChromiumRhodium Rhodium Second Constantan Constantan 5% Nickel Platinum PlatinumConstantan wires Aluminum Silicon

As appreciated by those skilled in the art, the junctions 108 representthe so-called “hot junctions,” which allow for relative temperaturemeasurement of the return electrode pad 30 since each of the “hotjunctions” are operatively connected to a “cold junction” (notexplicitly shown). During operation, a voltage is passed to each of thejunctions 108. Each junction 108 is interrogated individually, whereinthe switching of the interrogation signal is performed by a suitablemultiplexer (not explicitly shown) or a similar device. Theinterrogatory polling may be performed in a serial fashion and thepolling data is sent to the generator 20 or a control circuit disposeddirectly within the return electrode pad 30 as shown in FIG. 11 anddiscussed in more detail below.

A signal processor monitors the polling data for the junction 108 havingthe highest temperature. Thereafter, the temperature of the warmestjunction 108 is compared with the maximum safe temperature, which isfrom about 40° to about 45° C. Those skilled in the art will appreciatethat if the return electrode pad is of resistive type, heating thereofis more uneven than heating of capacitive type electrode pad. Therefore,spacing between junctions 108 is adjusted to accommodate various typesof return electrode pads.

FIG. 11 shows another embodiment of the return electrode pad 30 thatincludes a control circuit 51 disposed on flexible holding substrate 48.The control circuit 51 is coupled to the temperature sensing circuitsdisclosed in the above embodiments (e.g., temperature sensing circuits40, 90 and 100) and is configured to receive sensor signals therefrom.Other sensor circuits may be used in conjunction with the controlcircuit 51 and the discussion of the temperature sensing circuit 40represents one embodiment of the present disclosure.

In particular, the control circuit 51 analyzes the sensor signals andperforms similar functions as the sensor circuit 22. Since processing ofsensor signals occurs at the return electrode pad 30 this obviates theneed for running the interconnection wires 46 directly to the sensorcircuit 22. Consequently, isolation circuits for each of theinterconnection wires 46 are also no longer necessary. Placement of thecontrol circuit 51 at the return electrode pad 30 also provides areduction in amount of circuit components necessary for the generator 20and reduces high frequency leakage-to-earth referenced circuits.

The control circuit 51 includes an analog-to-digital converter 52, adigital-to-analog converter 54, a microprocessor 56, a DC-DC converter58, a serial transceiver 57, and an optical coupler 59. Those skilled inthe art will appreciate that the control circuit 51 may includeadditional circuit components, such as clocks, microcontrollers, coldjunction compensation with thermistor, resistors, capacitors,oscillators, field-programmable gate arrays, etc. The circuit componentsof the control circuit 51 are electrically insulated from thepatient-contacting surface 32 via the substrate 48. Further, since theholding substrate 48 includes metal traces deposited thereon, thecircuit components are bonded directly thereto and holding substrateacts as an electrical interconnect between the circuit components.

The control circuit 51 and the temperature sensing circuit 40 arepowered by the power source 50, which is coupled thereto via a powerline 60. The power line 60 includes one or more wires adapted totransmit lower voltage DC current. The DC-DC converter 58 adjusts thepower from the power source 50 to suit the circuit components of thecontrol circuit 51 and the temperature sensing circuit 40.

The temperature sensing circuits 40, 90 and 100 transmit correspondingvoltage, current and/or impedance signals through the interconnectionwires 46 to the control circuit 51. The control circuit 51 analyzes thesensor signals to determine a corresponding temperature value. Thesensor signals are initially passed through the A/D converter 52.Thereafter, the digitized sensor signals are analyzed by themicroprocessor 56 (e.g., multiplying the digitized sensor signal by apredetermined factor to arrive at a corresponding temperature value) toobtained processed data (e.g., temperature values). Those skilled in theart will understand that additional logic circuits may be included inthe control circuit 51, such as microcontrollers and field-programmablegate arrays, depending on the complexity of computations beingperformed.

The processed data is transmitted to the generator 20 for furtheranalysis via a data line 62. Prior to transmission, the temperaturesignals may be converted to analog signals for transmission via a serialdata transfer protocol. This is accomplished via the D/A converter 54.The serial transceiver 57 (e.g., universal asynchronousreceiver/transmitter) establishes serial communications with itscounterpart transceiver at the generator 20 and transmits the individualbits of processed data in a sequential fashion. The signals carrying theprocessed data are passed through the optical coupler 59 which isconnected to the data line 62. The optical coupler 59 isolates thecontrol circuit 51 from the RF supply by transmitting the signals acrossan isolation barrier. It is envisioned that the optical datatransmission methods utilizing fiber optics may be used in place of thedata line 62 to transfer data to the generator 20 from the controlcircuit 51. This eliminates electrical interference and RF leakage. TheRF energy is returned to the generator 20 via a return line 64. Thepower line 60, the data line 62 and the return line 64 are enclosedwithin the cable 8.

In embodiments, the analog sensor signals from the temperature circuits40, 90 and 100 may be transmitted to an analog MUX which is coupled to acounter (not explicitly shown). These circuit components are configuredto measure the voltage signal corresponding to the temperature readingsand transmit the processed data to the generator 20. Further, withreference to the embodiment of temperature sensing circuit 100 shown inFIGS. 10A-B, the multiplexer or similar device suitable for rapidswitching of interrogatory signals of the thermocouple matrix 102, mayalso be incorporated into the control circuit 51.

At the generator 20, the processed data is then transmitted to thecontroller 24 where it is further analyzed to determine appropriateaction. For instance, comparing temperature measurements with apredetermined temperature threshold and adjusting or terminating the RFenergy supply if the temperature measurement is larger than thethreshold.

The circuit components of the present disclosure are integrated directlyinto the return electrode pad 30. To minimize the cost of these pads, itis desirable to make the return electrode modular, wherein the circuitcomponents are reused from one procedure to the next. FIG. 12 shows across-sectional side view of a modular return electrode pad 110 whichincludes the circuit components (e.g., control circuit 51, temperaturesensing circuits 40, 90 and 100, etc.) disposed on a detachablesubstrate 112. The detachable substrate 112 is configured to temporarilyattach to disposable portion 114 (shown in assembled configuration inFIG. 12). More specifically, the disposable portion 114 includes thefoil layer 72 with the adhesive material layer 39 disposed on the bottomsurface thereof. A second adhesive material layer 116 is disposed on thetop surface of the foil layer 72. The second adhesive material layer 116is used to bond a removable release liner 118, which secures thedetachable substrate to the disposable portion 114. Further, the secondadhesive material layer 116 also acts as a heat conductor.

The detachable substrate 112 includes an insulative material layer 119at the bottom surface thereof and the bottom surface of the insulativematerial layer 119 is configured to selectively attach to the removablerelease liner 118. The insulative material layer 119 may be formed frompolyolefin, Mylar or other similar dielectric materials. Duringoperation, the release liner 118 is removed exposing the adhesivematerial layer 116 and the detachable substrate 112 is attached to thedisposable portion 114 via the insulative material layer 119 and theadhesive material layer 116. This configuration allows for thedetachable substrate 112 to be temporarily attached to the disposableportion 114 during electrosurgical procedures. Once the procedure iscomplete, the detachable substrate 112 is separated from the disposableportion 114.

While several embodiments of the disclosure have been shown in thedrawings and/or discussed herein, it is not intended that the disclosurebe limited thereto, as it is intended that the disclosure be as broad inscope as the art will allow and that the specification be read likewise.Therefore, the above description should not be construed as limiting,but merely as exemplifications of particular embodiments. Those skilledin the art will envision other modifications within the scope and spiritof the claims appended hereto.

1. An electrosurgical return electrode, comprising: a conductive pad including a patient-contacting surface configured to conduct electrosurgical energy; and a temperature sensing circuit coupled to the conductive pad, the temperature sensing circuit including a plurality of switching elements electrically connected in series, wherein each of the plurality of switching elements is a polymeric positive temperature coefficient overcurrent protection device configured to vary in impedance in response to temperatures changes, the temperature sensing circuit configured to receive an interrogation signal serially transmitted therethrough by a low voltage DC power source to measure an overall impedance value of the plurality of switching elements, wherein the measured impedance is indicative of the temperature changes.
 2. An electrosurgical return electrode according to claim 1, wherein the temperature sensing circuit is a flexible circuit.
 3. An electrosurgical return electrode according to claim 1, wherein each of the plurality of switching elements is configured to switch off to allow for increased impedance at different rates based on temperature.
 4. An electrosurgical return electrode according to claim 3, wherein each of the plurality of switching elements switches off at (i) a first rate at a temperature of about 40° C. and (ii) at a second rate at a temperature of about 45° C., the first rate being less than the second rate. 